Meeting the highest safety standards in all process steps is an indispensable requirement in the chemical industry. At BASF’s Swiss site in Kaisten, Switzerland there is a small but important building block in its highly efficient, global safety management. This is in the form of a guarantee that all processes are completely free of silicone oil. This is why traditional differential pressure transmitters can reach their limits when monitoring the levels during the production of additives for paints and plastics. They work by means of capillary lines filled with silicone oil, which – in the unlikely event of an accident – could come into contact with the medium. In contrast, VEGA electronic differential pressure measuring systems provide perfect safety with the innovative combination of ceramic and metallic measuring cells.
Plastics have been produced on an industrial scale for over 100 years. But anyone who spent their childhood in the 1970s or ‘80s knows that the plastics of yesteryear have little in common with today’s special plastics. The coveted Cowboy and Indian toy figures quickly turned yellow and some of the Lego bricks have become brittle. Plastics age due to thermo-oxidative degradation, i.e., under the influence of heat and oxygen. The fact that plastics today can be colourfast and elastic, crystal clear, high gloss, abrasion resistant or even heat resistant, is due to a wide variety of specialist additives.
The eye in the storm
The vapour pressure of liquids plays a decisive role in the production of such additives. Their precise measurement not only contributes to BASF’s highly efficient control of special processes, but also provides additional information on how process components can be optimised and plant availability increased.The more precise the values, the clearer the processes and the higher the potential for optimisation. There are extreme conditions, and aggressive substances and acids that can adhere to vessel walls, forming deposits, and high vacuum and steam levels. Through all this, the reliable measuring results of VEGA pressure transmitters play a key role.
Silicone oil-free measurement
While conventional differential pressure transmitters were previously used for level measurement, BASF’s measurement and control engineers in Kaisten have now decided to replace their instrumentation. They have switched their process control over to electronic differential pressure, which is silicone oil-free, without capillary lines, and therefore without mechanical pressure diaphragms. Thanks in no small part to the ceramic measuring cell, this system is ideal for vacuum and high temperatures. “The decision for electronic differential pressure on the basis of ceramic CERTEC measuring cells was obvious, in view of their reliability in withstanding abrasive substances. Our plant is completely automated and operates 24 hours a day, seven days a week,” says Markus Stoll, BASF’s head of specialist workshops, who together with his team, ensures the highest possible plant availability.
Strong demand for additives
BASF is one of the leading manufacturers of plastic additives, with production sites around the globe. These large-volume substances are produced by around 260 employees in Kaisten, from where they find their way into plastics and lubricating oils. Under precisely defined pressure conditions, the raw materials are converted into additives in several chemical reaction steps. A fully automatic process control continuously records each measured value, and ensures the exact dosing and the highly efficient operation of the plant. Reliability is crucial, because the end product is an essential intermediate for BASF. Downtime in this section has far-reaching consequences. “Even the smallest failure interrupts a complex process. In practical terms, this means weekend work and overtime for us. Complex readjustments and meticulous analysis would then become necessary,” explains Stoll.
No experiments, please
Actually, measurement and control engineers work according to the principle “never change a running system”. For processes to run smoothly, all production parameters must be precisely coordinated. It was only with the introduction of this new, oil-free pressure measurement technology, that some special features became apparent at BASF during the course of changeover. Distillation, including that of a reaction mixture under vacuum, is based on different boiling points of the liquids involved and their different vapour pressures. The desired separation effect depends on the composition of the liquids and the steam. For each differential pressure measurement, the pressure transmitters are precisely matched to the media to be monitored. In addition, special separating diaphragms are installed between the measuring cells and the process, in order to protect the cells from high temperatures or moisture.
One system for two extremes
The highly robust ceramic CERTEC measuring cell in the BASF vacuum reactor was ideal for the perfectly flush application in hot, aggressive solvents in the lower reactor section. In the upper reactor area, however, completely different conditions prevailed. Here, the extreme steam caused condensation in the measuring cell, leading to inaccuracies. Initially, the obvious solution was to use the differential pressure system based on metallic METEC measuring cells instead of ceramic measuring cells. Condensate and humidity no longer had any negative influence. “The upper measured value remained acceptable and highly accurate in all situations,” recalls Stoll. However, the high temperature of the solvent in the lower reactor area repeatedly caused measured value drifts.
Dream team for heat and condensate
It took expertise, a willingness to experiment, and, above all, hand-in-hand work between BASF’s Measuring and Control Technology department and Jürgen Feser, the key account manager at VEGA, until everything was running smoothly.
The surprising solution was to use an innovative electronic differential pressure measurement system with a combination of a ceramic and a metallic measuring cell. Both pressure transmitters record their respective values separately and calculate the differential in the master sensor. This is made possible by structurally identical instrument electronics behind the two very different measuring cells. They work perfectly, even in a mixed team. “For our application, this combination is a real dream team,” says Feser. “As far as I know, only VEGA offers this combination capability.” The bottom line result is one solution for two challenges. The ceramic measuring cell easily handles the high temperatures and aggressive substances in the lower reactor area (HP side). In the upper part (LP side), the metallic measuring cell performs perfectly in the presence of condensation.
Why is silicone oil-free so important?
In the 1970s, all major car companies rigorously declared their painting lines ‘silicone oil-free zones’. What was the reason? Time and again, mysterious paint defects occurred when painting vehicles. In the end, the unlikely cause was determined to be the hairspray used by some employees. Nowadays, considerably less hairspray is likely being used than on the ‘interesting perms’ of that decade. However, since then, the regulations for silicone oil-free production have become a standard requirement in many industrial production areas. They cover mechanical equipment and components such as pipes, pumps, valves or hoses, and suppliers are required to provide guarantees and documentation for their silicone oil-free production. BASF prohibits silicones in many production processes and areas at the Kaisten site.
Conclusion
Even the most ingenious manufacturing process for additives is only as good as the equipment of the production plant. In the case of BASF’s distillation reactors in Kaisten, the sensors of pressure transmitters really get the job done, even in the most inhospitable locations. In the lower part of the vessel this is under the high temperatures of the aggressive, adhesive liquid, and at the same time in the upper part of the vessel there is steam and heavy condensation. The perfect solution for the differing process conditions in a vessel turns out to be a simple variant of electronic differential pressure measurement from VEGA − the combination of a ceramic CERTEC and a metallic METEC measuring cell.
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